Error reading serial port with C #

Question

I have a system that makes a loop on all serial ports and send a command 'at' for them. These ports return 'ok' in Putty, but in the C# the MessageBox is empty. Why is this happening?

Putty: https://i.sstatic.net/6cbfk.png / C#: https://i.sstatic.net/soJc7.png

public partial class Form1 : Form
        {
            List<SerialPort> serialPort = new List<SerialPort>();

            private delegate void SetTextDeleg(string text);

            public Form1()
            {
                InitializeComponent();
            }

            private void Form1_Load(object sender, EventArgs e)
            {
                var portNames = SerialPort.GetPortNames();
                foreach (var port in portNames)
                {
                    SerialPort sp;
                    sp = new SerialPort(port, 9600, Parity.None, 8, StopBits.One);
                    sp.Handshake = Handshake.None;
                    sp.DataReceived += new SerialDataReceivedEventHandler(sp_DataReceived);
                    sp.ReadTimeout = 500;
                    sp.WriteTimeout = 500;

                    serialPort.Add(sp);
                    listPorts.Items.Add(port);
                }
            }

            private void listPorts_SelectedIndexChanged(object sender, EventArgs e)
            {

            }

            private void button1_Click_1(object sender, EventArgs e)
            {
                foreach (var sp in serialPort)
                {
                    // Open port
                    try
                    {
                        if (!sp.IsOpen)
                            sp.Open();

                        sp.Write("at\r\n");
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show("Error opening/writing to serial port :: " + ex.Message, "Error!");
                    }
                }
            }

            void sp_DataReceived(object sender, SerialDataReceivedEventArgs e)
            {
                Thread.Sleep(500);
                SerialPort sp = (SerialPort)sender;
                string data = sp.ReadLine();
                this.BeginInvoke(new SetTextDeleg(si_DataReceived), new object[] { data });
            }

            private void si_DataReceived(string data)
            {
                String retorno = data.Trim();
                MessageBox.Show(data);
            }
    }

Answer 1

Your modem response something like this: \n\rOK\n\r you should read all stream in the buffer or you should read 3 time the serial port buffer with ReadLine() method.